In the field of semiconductor production, food-products industry, and the like, the process of gas-liquid absorption, degasification, filtration, and so on uses a separation membrane module that houses in its housing a separation membrane element incorporating hollow-fiber-shaped porous separation membranes. The separation membrane element is formed by bundling a plurality of hollow-fiber-shaped porous separation membranes together and by sealing end portions of the membranes using a membrane-sealing portion made of resin to unite the membranes with the membrane-sealing portion.
The sealing of the hollow-fiber-shaped porous separation membranes in the production of the separation membrane element is conventionally performed by the following process. First, the end portions of the hollow-fiber-shaped porous separation membranes are set in a mold. Second, a resinous liquid (which means a resin in a liquid state, and the same is applied to the following explanation) is cast into the mold to immerse the end portions of the hollow-fiber-shaped porous separation membranes in the resinous liquid. Finally, the resinous liquid is cured to form the membrane-sealing portion. (Hereinafter, the foregoing process is sometimes referred to as the immersion forming process.) FIG. 3 is a sectional view showing a condition of the above-described casting (immersion). In FIG. 3, the sign 35 indicates one of the hollow-fiber-shaped porous separation membranes, and the sign 35′ indicates the hollow portion of the membrane. FIG. 3(a) shows a condition in which an end portion 32 of the hollow-fiber-shaped porous separation membrane 35 is immersed in a resinous liquid 33 that is cast in the mold (not shown).
When the casting (immersion) is performed, the opening of the hollow portion at the end of the hollow fiber is closed by a method of sealing, tying together with other hollow fibers, or the like in advance to prevent the resin from flowing into the hollow portion of the hollow fiber (hereinafter, the closed portion is referred to as the opening-closed portion). After the curing of the resin, the end portion of the hollow fiber is severed together with the cured resin (the resin's portion in the vicinity of the end portion of the hollow fiber) to expose at the end the opening of the hollow portion. The sign 34 in FIG. 3(a) indicates the opening-closed portion at the end of the hollow fiber. FIG. 3(b) shows a condition in which the opening-closed portion 34 at the end of the hollow fiber is severed together with the cured resin in its vicinity (the portion enclosed by the frame “m” in FIG. 3(a)). Thus, the membrane-sealing portion is formed in which the opening of the hollow portion 35 is exposed.
According to the immersion forming process, the membrane-sealing portion can be formed through only a few steps. Therefore, this process is desirable in view of the productivity. In this process, when the hollow-fiber-shaped porous separation membrane is immersed in the resinous liquid, the resinous liquid 33 penetrates into numerous fine pores in the end portion 32 of the hollow-fiber-shaped porous separation membrane (the individual pores are not shown). By curing the cast resinous liquid 33 and the pore-filling resinous liquid 33, an anchoring effect is exerted between the resinous liquids. This anchoring effect improves the bonding ability between the hollow-fiber-shaped porous separation membrane and the membrane-sealing portion, enabling the two members to unite reliably (Patent Literature 1).
As the material for constituting this type of separation membrane module, it is desired to use a material having chemical resistance because the module sometimes treats corrosive gas or liquid. For example, a porous fluororesin, which has high chemical resistance, is widely used for the material of the hollow-fiber-shaped porous separation membranes. In addition, the membrane-sealing portion is also desired to be formed of a material having high chemical resistance because it is used in contact with the gas and liquid to be treated.
The membrane-sealing portion is conventionally formed of urethane resin or epoxy resin. However, these resins have lower chemical resistance than that of fluororesin. Consequently, there has been a problem in that when corrosive gas or liquid is treated, the membrane-sealing portion tends to deteriorate due to the contact with the gas or liquid to be treated at the time the separation membrane module is used. To solve the problem, engineers have proposed the use of thermoplastic fluororesin, which is a material having high chemical resistance, such as a tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer (PFA) (Patent Literature 2).
Patent Literature 1: the published Japanese patent application Tokukaihei 3-106422
Patent Literature 2: the published Japanese patent application Tokukaihei 9-290138.